Solvus Temperature Research Articles

Continuous γ′ precipitation in directionally solidified IN738 LC alloy

A microstructural characterisation of continuously precipitated γ′ particles in IN738 LC alloy directionally solidified at various cooling rates has been carried out. With decreasing cooling rate there is a trend for the γ′ morphology to vary from rounded particles to irregular cuboids and, finally, a clusterlike formation. In the interdendritic regions, microsegregation leads to an increase in the γ′ solvus temperature, and γ′ precipitation begins at higher temperatures than in the dendrite cores, resulting in differences in particle size and morphology. Observations on the effects of solution treatment and aging on the development of γ′ dispersions are also reported.

Rapid exsolution behaviour in the bornite–digenite series, and implications for natural ore assemblages

Abstract Intermediate compositions along the bornite–digenite join exsolve during quenching from above-solvus temperatures. This involves vacancy clustering and cation ordering processes, and is facilitated by fast cation diffusion rates in the presence of a large (10–25%) metal vacancy population. Samples of six different compositions across the bornite–Cu9S5 join, synthesised from component elements in sealed quartz capsules, were water-quenched from 600°C and analysed using high-resolution neutron powder diffraction (HRPD). Time-of-flight spectra measured at room temperature showed all intermediate compositions had exsolved into mixtures of bornite and low digenite with a 5.0a superstructure. No evidence for the presence of any other phase was found. Variations in the lattice parameters of the exsolved bornite phase were observed for different bulk compositions across the join, and ascribed to variations in the degree of order. Bornite exsolved from digenite-rich compositions may not be fully ordered due to the much lower solvus temperatures at the Cu-rich end of the solid solution. As only slight differences were observed between the diffraction patterns of a visibly exsolved and a rapidly quenched sample of the same bulk composition, the formation of optically-visible exsolution lamellae on {100} is ascribed to a process of coalescence of sub-microscopic domains initially formed during the quenching process. The rapid kinetics of exsolution at geologically low temperatures, explains the lack of authenticated natural occurrences of intermediate compositions in the solid solution in nature, and the limited degree of stoichiometric variation observed in end-members.

Effect of thermal cycles on ferrite content of austenitic stainless steel

The properties of austenitic stainless steel weld metals are strongly influenced by the duplex structure of austenite and ferrite. The objective of the present study was to investigate the effect of thermal cycles on the ferrite content in austenitic stainless steel weld metal, and to explore the ferrite–austenite solid state transformation of austenitic stainless steel weld metal. The results indicate that the peak temperature, cooling rate, and number of thermal cycles have a significant effect on ferrite content in the austenitic stainless steel weld. As the cooling rate increases, or the number of thermal cycles decreases, the amount of retained ferrite is increased at room temperature. New δ ferrite phase can be formed if the reheating peak temperature is above the optimum γ solvus temperature of austenitic stainless steel, and this new δ ferrite can be retained under rapid cooling conditions.

High-Strain Rate Superplasticity in Three Contrasting Fine Grained Aluminum Alloys

AbstractThe superplastic properties and microstructures of three contrasting fine grained aluminium alloys were investigated. These included (i) a powder metallurgy MMC, (ii) a severely deformed spray cast alloy, and (iii) the Zn-22%Al eutectoid alloy. The results showed some differences in the details of behaviour between the alloys. One of these was the presence of true work hardening, associated with dislocation activity, in the MMC, and its absence in the microduplex Zn-Al eutectoid alloy. In addition, the powder route MMC had the high threshold stress (up to 10MPa) commonly encountered in such materials, whereas this was not the case with the cast and severely deformed alloy, indicating that the threshold stress was associated with the presence of ceramic particles in the MMC (oxide + reinforcement). In all the alloys, however, the m value tended to increase with temperature, and this led to a corresponding increase in elongation with temperature until the microstructure became destabilised. In the two predominantly single phase alloys studied this destabilisation corresponded to the solvus temperature. The constantly changing m value indicates that there is no unique value of this parameter. The same was found to be true of the grain size exponent, p. The direct interpretation of apparent activation energies in terms of simple physical processes should be made with care in the light of the present results, as (i) the microstructures of two of the alloys were found to change continuously and significantly with small changes in temperature, and (ii) the fact that m is a function of temperature necessarily implies that the apparent activation energy is a function of stress.

Ｔｉ‐６Ａｌ‐４Ｖのミクロ組織と機械的性質におよぼすＳｉ添加の影響

The effects of silicon addition on microstructures and tensile properties for Ti-6Al-4V alloy casting were investigated. And the possibility of microstructure control for Ti-6Al-4V-Si alloys by hot-working was discussed. Fine blocky Ti5Si3 intermetallic compounds were precipitated during solidification and cooling at silicon bearing alloys. When silicon contents exceeds 1.96%, plate-like compounds were observed around grain boundaries. Lamella structures were formed at 2.75%Si alloy by eutectic reactions from liquid to beta and Ti5Si3. The solvus temperature of Ti5Si3 was measured by microstructure's observation after solution treatment. Its temperature rose rapidly by silicon addition, and reached to 1400°Cat 2.75% silicon bearing alloy. Lamella structures were solved in beta matrix at equilibrium condition. Silicon addition is effective for beta-grain refinement. Tensile strength was increased rapidly by silicon addition to 0.93%, however, when silicon contents exceeded 1.96%, unstable brittle fracture at a low stress level occurred due to grain boundary precipitates. Fine and homogeneous spheroidized precipitates were obtained by soaking and hot working. The combination of strength and ductility was improved by soaking and hot working. Good combination of strength and ductility was especially obtained in 0.90% silicon bearing alloy which was hot-worked at 950°C. Tensile strength and elongation of that alloy were 1338MPa and 10%. Refinement of micro-structure and improvement of mechanical properties by thermo-mechanical treatment will be possible furthermore.

The effect of flow localization and shear bands development on the structure and mechanical behavior of Cu-4.98 wt.% Ti alloy

Hot compression tests were performed on solution treated (ST) Cu-Ti alloy deformed within a temperature range of 673 K - 1173 K at constant strain rate of 1.4×10 -4 s -1 and 6.9×10 -3 s -1 . Shear banding, dynamic precipitation and dynamic coarsening have operated simultaneously with different effectiveness depending on the temperature and used strain rate. At low temperature, 673 K, intergranular cracks and flow localization were found to be responsible for the material fracture at e 1 0.1 - 0.2. At 873 K and above, a flow stress maximum was followed by monotonic flow softening range. Initial hardening range has resulted from both strain and precipitation hardening. Structural observations leads to the conclusion that an interaction of localized flow and discontinuous precipitation within shear bands was responsible for effective flow stress decrease at larger strains. At high deformation temperature, the flow softening of ST samples has resulted from cellular growth and combined effect of flow localization and discontinuous precipitation within sheared area. At high strain rates, the shear banding was found to proceed the precipitation and the initial flow softening has resulted from shear bands development rather than discontinuous precipitation. During deformation at 873 K - 1073 K and low strain rate, discontinuous precipitation was observed since the beginning of the tests and the localized flow was intensified due to particles dynamic coarsening within shear bands. The structural flow localization effects were limited for overaged (OA) and hot deformed samples. During deformation above the solvus temperature, dynamic recrystallization of the material was observed for both ST and OA samples.

Microstructure of hot-deformed Cu–3.45 wt.% Ti alloy

Hot compression tests at temperatures ranging from 673 to 1173 K were carried out on solution-treated Cu–3.45 wt.% Ti at a constant true strain rate of 1.4×10 −4 s −1 . According to literature data, an effective hardening of Cu–Ti alloys results from continuous precipitation of small metastable β ′ particles of Cu 4 Ti-phase which follows the spinodal decomposition of the solid solution during static aging of a solution treated alloy. This process was also responsible for an effective hardening of the material during hot compression of the solution treated alloy in the early stages of the test. At low deformation temperatures (673–773 K), shear banding and growth of intergranular cavities were found to be responsible for a flow stress decrease and for sample fracture at large strains. During hot deformation at intermediate temperatures (873–973 K), softening was also found to follow the flow stress maximum. According to the literature, the discontinuous precipitation of stable β particles of Cu 3 Ti-phase results in a decrease of strength during static aging, i.e. an overaging effect. However, during hot deformation of a solution treated alloy in this temperature range dynamic precipitation coarsening within shear bands is also observed to proceed in competition with cellular growth of β particles. Phase transformation of fine-scale precipitates into colonies of coarse Cu 3 Ti particles produces a ‘soft’ structural component within the hardened matrix. This as well as intensive shear banding was responsible for the effective flow softening of the material. Moreover, discontinuous precipitation at grain boundaries is found to reduce the growth of cavities and to delay intergranular fracture. Thus, samples deformed at temperatures from 973 to 1023 K did not fracture in the deformation range used in the experiments. It is suggested that intergranular crack growth is retarded by dynamic recovery within shear bands as well as by dynamic recrystallization above the solvus temperature.

Kinetics of discontinuous precipitation and dissolution in Cu–Ag alloys

The kinetics of discontinuous precipitation and dissolution of the cellular precipitate have been studied in Cu-3 at.% Ag and Cu-4 at.% Ag alloys. The growth rates of the cells were measured using optical microscopy. The interlamellar spacings of the primary cells and compositions of the depleted matrix were measured using scanning electron microscope and X-ray diffraction, respectively. The Cu–Ag alloys were observed to decompose into a lamellar structure consisting of alternate lamellae of the α (Cu-rich) and β (Ag-rich) phases when a solid solution of the alloy was aged below the solvus temperature. The rod shaped morphology of the β phase dispersed in the matrix of α was observed at all temperatures. The primary cell growth data were analysed using the theories of Cahn, Hillert, Sundquist, Turnbull and Petermann and Hornbogen. From the diffusivity values, it has been shown that the growth of primary cells occurs by the diffusion of Ag along the grain boundaries. The results are consistent with the diffusivity values reported in the literature in the same temperature range. The discontinuous dissolution of the primary cells occurred above the solvus temperature as well as 30–40 K below it. The dissolution occurred primarily at the prior positions of grain boundaries as well as at the primary cells intersection, at least in the initial stages of dissolution. The discontinuous dissolution occurred by the diffusion of Ag along the interface boundary between the primary and dissolution cells. The diffusivity and mobility values obtained during dissolution are one order of magnitude smaller than those obtained for the discontinuous precipitation. This has been explained by volume diffusion ahead of the interface and not all of the free energy being used to drive the interface boundary.

Petrogenesis of dolomitic marbles from Rongcheng in the Su‐Lu ultrahigh‐pressure metamorphic terrane, eastern China

Marbles are interbedded with biotite–hornblende gneiss in the Rongcheng area, Su‐Lu ultrahigh‐pressure (UHP) metamorphic terrane, eastern China. Both marble and gneiss include UHP eclogite layers and boudins. Seven dolomitic marbles were selected for petrologic investigation. Dolomitic marbles have assemblages of major constituent minerals: (i) Mg–Cal + Dol + Ol; (ii) Mg–Cal + Dol + Di + Ol; (iii) Mg–Cal + Dol + Di; (iv) Mg–Cal + Dol + Ti–Chu; (v) Mg–Cal + Ti–Chu + Di; and (vi) Mg–Cal + Dol + Ti–Chu + Di + Ol. Titanium–clinohumites of (iv) and (v) contain 3∼4 wt% in TiO2, but those in (vi) are < 3 wt% and are regarded as later‐stage replacement products from olivine. Assemblages indicate uneven distribution of XCO2 within an individual sample. Schreinemakers' analysis of assemblage Arg (Cal) + Dol + Chu + Di + Fo in the system CaO–MgO–SiO2–CO2–H2O with thermodynamic calculations indicates that Fo‐ and Chu‐bearing assemblages are stable at very low XCO2 conditions, whereas Arg (Cal) + Dol + Di is stable at relatively higher XCO2 conditions. When XCO2 conditions are extremely low, Fo‐bearing and Chu‐bearing assemblages are stable at T > 800 °C and P > 2.2 GPa (XCO2 = 0.01), and T > 730 °C and P > 2 GPa (XCO2 = 0.005). If such extremely low XCO2 is possible, the assemblages (i–v) could be regarded as products at UHP conditions excepting Mg–calcite transformed from aragonite + dolomite. Calcite–dolomite intergrowths are common in these assemblages. Mg–calcite inclusions were found in Ti–clinohumite of assemblage (iv) with well developed radial fractures. The maximum XMgCO3 value of 0.111 yields a solvus temperature of 698 °C. If this Mg–calcite formed from aragonite + dolomite, solvus thermometry gives a minimum P–T point along the equilibrium curve of the reaction aragonite + dolomite = Mg–calcite during the decompressional stage; T = 680 °C and P = 1.9 GPa. Along both the prograde and retrograde P–T paths for UHP dolomitic marbles, changes in model proportion among carbonate phases including aragonite, dolomite and Mg–calcite were proposed on the basis of the phase relations in CaCO3–CaMg(CO3)2. Aragonite + dolomite is a stable assemblage during the prograde stage. Appearance of the eutectoid point between aragonite and dolomite in the phase diagram plays an important role for phase changes in this system. After peak‐P, the following phase changes are predicted for the bulk compositions < XMgCO3 at the eutectoid point during the decompressional stage: (i) Arg + Dol; (ii) Arg + Dol → Mg–Cal (eutectoid composition); (iii) Arg + Mg–Cal (XMgCO3 < eutectoid composition); (iv) Mg–Cal; (v) Mg–Cal (low‐XMgCO3) + exsolved Dol.

Ｍｇ‐重希土類元素‐Ｚｒ合金の高温変形特性および鍛造性

The aim of this study is to determine optimum forging conditions for Mg–Gd–Nd–Zr and Mg–Gd–Y–Zr alloys. Firstly, homogenization temperature of these alloys was altered at the solvus temperature, above and below that temperature to evaluate the effect of grain size and remained compound on high temperature deformation characteristics by conducting high temperature tensile and compression tests. Consequently, an increase in elongation is obtained in specimens with both a small amount of compound and a small grain size. However, the prevention of grain coarsening is found to be a more important factor for the increase in elongation of these specimens than the amount of compound. As a next step, these specimens controlled the grain size more than 65 μm were used for die forging into a piston shape under the conditions of a die speed of 0.36 to 0.41 m/s, a specimen temperature of 500°C and a mold temperature of 300°C. Applying these optimum forging conditions leads to the production of die-forged pistons with an excellent surface finish and without internal cracks.

Heat treatment effects on SiC fiber

The Wright Laboratory Materials Directorate at Wright-Patterson AFB has been spearheading the development and evaluation of a new class of metal matrix composites based upon continuous SiC fiber reinforcement of orthorhombic phase containing titanium aluminide matrices. These composites (O TMCs) will be subjected to thermal exposures during primary and secondary component processing, and possibly also during heat treatments to optimize matrix-dominated mechanical performance. Such thermal excursions must not degrade the SiC fiber reinforcement, hence compromising resulting composite properties. Therefore, the effects of heat treatment on the room temperature tensile strength of continuous SiC fibers were studied. The fibers examined included: Trimarc 1®, SCS-6, Ultra SCS and an experimental large diameter version of Ultra SCS. The fibers were heat treated below and above the beta solvus temperature of the orthorhombic matrix alloy utilized for this study, Ti-22Al-23Nb (at%). The fibers were evaluated for ambient temperature tensile strength in the following conditions: (1) as-received: (2) heat treated in vacuum; and (3) consolidated into Ti-22Al-23Nb, heat treated in vacuum, and chemically extracted. Fiber microstructure and fracture analysis was accomplished via secondary scanning electron microscopy (SEM). Chemical reactions between fiber core and the SiC, and between the SiC fiber and the Ti-22Al-23Nb matrix, were also studied by SEM.

Effect of deformation on low-temperature deuteride precipitation in single crystal niobium

Small-angle neutron scattering (SANS) measurements of low concentration, single crystal Nb - D alloys have been performed to characterize the effect of deformation on deuteride phase formation. The SANS response from the deuteride phase formed at low temperature in the undeformed matrix was consistent with a spherical particle morphology, as expected from a large body of published TEM studies. In addition, a significant hysteresis was observed in the SANS response versus temperature from the undeformed sample material. The hysteresis is attributed to the irreversible plastic deformation required to nucleate the incoherent phase. Deformation produced by unidirectional rolling to 50% thickness reduction altered the deuteride phase transformation characteristics in a fundamental way. The SANS response was highly anisotropic, indicative of an oriented, anisometric particle morphology. Metallographic analysis of deformed specimens electropolished below the solvus temperature confirmed the presence of large deuteride plates with an orientation consistent with the anisotropic SANS response. Deformation also eliminated the temperature hysteresis observed in the undeformed material. A cellular dislocation substructure was found in TEM characterization of the deformed Nb single crystal material. The plate-like particle morphology in the deformed material is attributed to an interaction between the precipitating deuterium and the cellular dislocation substructure. The elimination of the temperature hysteresis is consistent with an interaction between the deuterium and regions of high dislocation density facilitating heterogeneous nucleation.

The density discontinuity associated with the precipitation of the second phase in Pb + 7.45 at.% Sn alloy

Associated with the precipitation of an Sn-rich phase from a Pb + 7.45 at,% Sn alloy during cooling at a constant rate from an elevated temperature well above the solvus temperature, there is a measurable and precipitous decline in density which begins at a temperature slightly below the solvus temperature for this alloy. This decline takes place over a limited temperature range and is attributed to the formation of excess vacancies created during the nucleation of the precipitation reaction. A similar formation of excess vacancies has been observed using positron annihilation spectrometry in a similar alloy during cooling through the solvus temperature. During this decline in density, the concentration of these vacancies much exceeds the equilibrium vacancy concentration for this alloy. The excess vacancies are unstable and, with continued cooling, quickly anneal out over a narrow temperature range allowing the versus temperature curve to regain linearity.

The determination of solute clusters in dilute aluminum alloys using strain rate sensitivity

Precise, apparent activation volumes, V′, are determined at 78 K for aluminum solvent systems containing mostly Fe, Cr, Cu or Mg in solution. The relationship between 1/V′ and the flow stress correlates to the amount of solute in the aluminum matrix. It appears that only Fe solute manifests solute-defect interaction effects which approximate a random solid solution. All other solute elements exhibit a much reduced distribution of strain centres, suggesting that these solutes are clustered even though the dilute alloys were solutionized at temperatures considerably above the solvus temperature. Stress equivalence of the apparent activation volume is observed over a range of yield stress, σ 0, as V′ ∝ σ 0 −0.95.

Discontinuous precipitation in a Cd-6 at.% Ag alloy

Discontinuous precipitation (DP) in a Cd-6 at.% Ag alloy has been investigated for the first time. The precipitate phase maintains a lamellar morphology and statistically constant interlamellar spacing under a given isothermal condition in the temperature range studied (333–523 K). The interlamellar spacing increases with an increase in isothermal temperature. The reaction front velocity registers a typical C-curve variation with the inverse of temperature. The reaction rate is maximum at 470 K. The predicted upper limit of DP occurrence in this alloy is 23 K lower than the concerned equilibrium solvus temperature. Continuous precipitation accompanies DP at all temperatures, especially beyond a certain time, and adversely affects the growth kinetics of DP colonies by reducing the local chemical driving force and/or posing physical hindrance to the reaction front migration. An extensive kinetic analysis of DP using the models by Turnbull, Aaronson and Liu, and Petermann and Hornbogen has yielded the grain boundary chemical diffusivity data in Cd-6 at.% Ag for the first time, the activation energy of which lies in the range 55–77 kJ/mol.

Precipitation hardening in nickel based superalloys: effect of alloying

The growth kinetics of γ′ precipitates in Ni based wrought superalloys was investigated, using a computer aided X-ray analysis method. It was found that the growth process obeys the Lifshitz–Slyozov–Wagner law. Regression formulae describing the liquidus, solidus, and γ′ solvus temperatures as functions of the overall composition were established. The heat treatment window as a function of composition and its effect on homogenisation and retained hardness were determined. The high retained hardness of homogenised Al + Ti rich alloys hinders their commercial use as Ni base wrought superalloys.MST/3206

Precipitation hardening in nickel based superalloys: effect of alloying

The growth kinetics of γ′ precipitates in Ni based wrought superalloys was investigated, using a computer aided X-ray analysis method. It was found that the growth process obeys the Lifshitz–Slyozov–Wagner law. Regression formulae describing the liquidus, solidus, and γ′ solvus temperatures as functions of the overall composition were established. The heat treatment window as a function of composition and its effect on homogenisation and retained hardness were determined. The high retained hardness of homogenised Al + Ti rich alloys hinders their commercial use as Ni base wrought superalloys.MST/3206

Processing window of the near-α-titanium alloy TIMETAL-1100 to produce a fine-grained β-structure

The microstructural evolution and mechanical properties of TIMETAL-1100 were studied for different thermomechanical treatments. The existence of a processing window between the β-transus temperature and the silicide solvus temperature allows a lamellar microstructure to be produced with relatively small transformed β-grains. The microstructure could be varied over a wide range using three solution heat treatment temperatures, below the β-transus, between the β-transus and the silicide solvus and above the silicide solvus, followed by four different cooling rates. Silicide precipitation was also studied, in particular its influence on the microstructural evolution of TIMETAL-1100. The room temperature tensile properties of TIMETAL-1100 were influenced significantly by the different thermomechanical treatments and silicide precipitation.

Precipitation in Some Dilute Al-Ge-Si Alloys

Six Al-Ge-Si alloys with a constant solute content of 1.3 at.% have been investigated with respect to solvus temperatures, solid solubility at 400 °C and precipitation behaviour at 160 °C. Solvus temperatures were determined both by measurements of electrical resistivity and by scanning electron microscopy (SEM). The results of the two methods were fairly consistent. Specimens annealed at 400 °C were studied by SEM. The area fractions of the precipitates were measured, and used for calculations of the solid solubility of the alloys at this temperature. From the results obtained, one has proposed a most likely shape of the isothermal sections of the Al-rich solvus surface. Age hardening experiments showed an improved hardness in some of the ternary alloys, as compared to the binaries. However, it seems like all alloys are equally dependent on the quenched-in vacancies to facilitate nucleation of precipitates.

Influence of subsolvus thermomechanical processing on the low-cycle fatigue properties of haynes 230 alloy

Strain-controlled low-cycle fatigue tests have been conducted in air at elevated temperature to determine the influence of subsolvus thermomechanical processing on the low-cycle fatigue (LCF) behavior of HAYNES 230 alloy. A series of tests at various strain ranges was conducted on material experimentally processed at 1121 °C, which is below the M23C6 carbide solvus temperature, and on material fully solution annealed at 1232 °C. A comparative strain-life analysis was performed on the LCF results, and the cyclic hardening/softening characteristics were examined. At 760 °C and 871 °C, the fatigue life of the experimental 230/1121 material was improved relative to the standard 230/1232 material up to a factor of 3. The fatigue life advantage of the experimental material was related primarily to a lower plastic (inelastic) strain amplitude response for a given imposed total strain range. It appears the increase in monotonic flow stress exhibited by the finer grain size experimental material has been translated into an increase in cyclic flow stress at the 760 °C and 871 °C test temperatures. Both materials exhibited entirely transgranular fatigue crack initiation and propagation modes at these temperatures. The LCF performance of the experimental material in tests performed at 982 °C was improved relative to the standard material up to a factor as high as 2. The life advantage of the 230/1121 material occurred despite having a larger plastic strain amplitude than the standard 230/1232 material for a given total strain range. Though not fully understood at present, it is suspected that this behavior is related to the deleterious influence of grain boundaries in the fatigue crack initiations of the standard processed material relative to the experimental material, and ultimately to differences in carbide morphology as a result of thermomechanical processing.